It is well known that excessive internal pressure within the human eyeball is a component of glaucoma, a disease of the eye. This disease accounts for a significant percentage of all blindness. Surveys have shown it to be present and unrecognized in a significant number of people, particularly people over the age of 40 and even more so for people over the age 50.
It is also known that where the presence of glaucoma can be identified at an early stage, damage to the eye and subsequent blindness can be arrested. Appropriate medication and surgery can serve to arrest the progress of the disease so that useful vision is retained.
In view of the fact that glaucoma is widespread, numerous proposals have been made for measuring the internal eyeball pressure. Many of these are complex precision instruments, which are expensive, and which require elaborate clinical settings for their operation. Typically, such instruments apply an amount of force to the eyeball, sufficient to allow an objective measurement of specific flattening (applanation) or indenting (indentation) of the surface of the eye. The amount of force required to achieve a certain applanation or indentation is correlated with the intraocular pressure measured internally, and usually expressed in mm of mercury.
Conventionally, the clinical instrument involved has some element which is applied directly to the cornea of the open eye to measure the applanation or indentation of the cornea. In view of natural human reflexes, this requires a topical anesthetic. The equipment is complex and costly and requires a trained and sophisticated technician to operate it. Other proposals have been made, and the following patents list proposals known to the applicant: U.S. Pat. Nos. 1,637,421; 1,661,718; 2,656,715; 5,176,139; and 5,197,473; French Patent 2,542,603; and Russian Federation Patents 2,004,187 and 457,466.
The Lipschutz U.S. Pat. No. 1,637,421 is a pressure indicator. It is not concerned with measuring eyeball pressure, but rather it is concerned with applying pressure to other parts of the human body. It is based on the well known phenomenon that sensitivity to pressure of an area of the body is an indication of disease. More particularly, it relies on the fact that the progress of the disease is related to the sensitivity of an associated area of the body. As such, it provides a device enabling the pressure applied to a particular area to be measured, so this pressure can be correlated with the progress of the disease. No clear directions are given, with regard to applying this technique to the human eye. Measuring pressure in the human eye presents unique and difficult problems, as compared to other parts of the anatomy. As the human eye is sensitive and delicate, everyone has a strong, natural reflex to close their eyes, if any attempt is made to touch the eye. This Lipschutz patent does not address this issue.
A hardness testing device is disclosed in U.S. Pat. No. 1,661,718 which is of marginal relevance.
An ocular tension indicator is disclosed in the Tolman U.S. Pat. No. 2,656,715. However, this requires the eyeball to be contacted. It relies upon relative axial displacement of different components of known, set weight, to determine the pressure within the eye. As such, it appears to be a delicate, precision instrument. Since it must contact the naked eye, it cannot be used outside of a clinical setting.
The two Fedorov U.S. Pat. Nos. 5,176,139 and 5,197,473 disclose an ocular tonometer and a related method. This relies on a somewhat unique technique where a ball is permitted to fall freely onto an eyelid-covered cornea. The kinetic energy of the ball deforms a cornea. The amount of the ball rebound varies depending upon the amount of intraocular pressure and this is judged against the height of the ball rebound. This technique would appear difficult to carry out, since it depends upon judging the height of the ball rebound.
Russian Patent 457,466 discloses an intraocular pressure transducer. This relies upon a Hall effect generator. Weights determine the penetration force of a plunger, whose displacement is sensed by the Hall effect generator with an output proportional to the displacement. Russian Patent 2,004,187 discloses an eye tonometer having a hollow cylindrical body with tips and working end face surfaces. It is not clear how this device is intended to work. In any event, it is again intended to be applied to the naked eyeball, which again would require the application of a topical anesthetic in a clinical setting.
U.S. Pat. No. 3,992,926 discloses an applanation tonometer incorporating a transducer for generating a digital readout of pressure. In use, the tonometer device measures differential pressure changes during the intraocular pressure measurement process. The transducer identifies a specific pressure change which corresponds with known tonograph results.
PCT patent application PCT/CA97/00341 discloses an applanation tonometer for measuring intraocular pressure within the eye. The tonometer includes a main body and a plunger, wherein the plunger is brought up against the eyelid of a closed eye and the main body is pushed towards the eyelid, which displaces the plunger into the main body. Displacement of the plunger relative to the main body is terminated when a pressure phosphene is detected. The displacement of the plunger is determined by a marker which is also indicative of the intraocular pressure within the eye.
Now, one of the problems with measuring intraocular pressure is that it can vary during the course of the day, and even from hour to hour. Accordingly, it is highly desirable to provide some simple technique for measuring this pressure. This technique should enable an ordinary person to measure the intraocular pressure within their eyes, without requiring complex expensive equipment, without requiring attendance at a clinic or the like, and without requiring the time of highly trained clinical staff. Also, it is highly desirable to record diurnal (occurring in a 24-hour period) variations in intraocular pressure overnight. It has been found that glaucoma patients have much higher diurnal intraocular pressure variations (8–11 mm Hg) in comparison to healthy subjects (about 4 mm Hg variations).